1. Technical Field
The present invention relates to an oscillator circuit that generates an oscillation signal and a test apparatus that tests an electronic device.
2. Related Art
A phase locked loop (PLL) circuit is conventionally known as a circuit for generating an oscillation signal. According to the PLL circuit, a reference signal and an oscillation signal generated by a voltage controlled oscillator are compared with each other in terms of phase, and the frequency of the oscillation signal generated by the voltage controlled oscillator is controlled based on the result of the comparison. The frequency of the oscillation signal is equal to an integral multiple of the frequency of the reference signal, and is controlled at the timing of each edge of the reference signal so that the oscillation signal is synchronized to the reference signal. The voltage controlled oscillator of the PLL circuit is typically formed by using a ring oscillator circuit, an LC resonator circuit or the like.
As mentioned above, the voltage controlled oscillator, which is configured by using a ring oscillator circuit or LC resonator circuit, generates an oscillation signal based on positive feedback or the like. This poses a problem that a phase noise attributed to the voltage controlled oscillator is gradually accumulated. To be specific, since the oscillation signal is compared to the reference signal in terms of phase, the mean value of the phase of each cycle of the oscillation signal is synchronized to that of the reference signal, but the phase jitter component of each cycle is accumulated. For example, the amplitude of the phase jitter component gradually increases with the mean value of the phase of each cycle of the oscillation signal being positioned in the middle.
In view of the above, an advantage of some embodiments of the present invention is to provide an oscillator circuit and a test apparatus that can solve the above-described problem. This advantage is achieved by combining the features recited in the independent claims. The dependent claims define further effective specific example of the present invention.